1. Field of the Invention
The present invention relates to electrical heating cables that use thermal switches to regulate zone-type heating elements.
2. Description of the Prior Art
Flexible, elongated electrical cables have been used commercially for many years for heating pipes, tanks, valves, vessels and for a variety of other applications. The heating cables maintain the temperature of fluids in pipes or equipment and prevent freezing.
Two significant types of electrical heating cable are currently available. The first is a constant wattage heater of the type depicted in FIG. 1. A constant wattage heater typically comprises two conductors connected to a power supply with a number of resistive elements aligned in parallel with each other and connected to the conductors. Electrical current is supplied to the conductors and passes through the resistive elements to generate heat. Temperature control of a constant wattage heater is generally achieved by means of an external thermostat which delivers or interrupts current to the entire cable based on the temperature of the pipe or the temperature of the cable.
Providing a single external control for the entire cable has significant shortcomings. In many applications, heat requirements may differ significantly for various points on the cable. A constant wattage heater, however, generates heat relatively uniformly along its length in response to a single thermostat control, and has the potential to provide too much heat for certain areas and not enough for others. If the thermostat is not placed in a representative location, the cable may overheat or the fluid may cool below the desired temperature. Further, the high-current controllers used in conjunction with constant wattage heaters may fail in certain high-wattage conditions. Failure of the controller can cause the cable to overheat if failure occurs in the on position, or interrupts heat generation for the entire cable if failure occurs in the off position.
The second major type of heating cable is the self-limiting or self-regulating type, an example of which is shown schematically in FIG. 2. Like a constant wattage cable, a typical self-regulating heating cable comprises a pair of conductors connected to a power supply and has either a number of discrete positive temperature coefficient (PTC) resistive elements connected in parallel with each other, as shown in FIG. 2, or a strip or web of PTC conductive polymer connecting the conductors. Instead of requiring an external thermostat like the constant wattage heaters, the PTC material or elements control the current flow to the resistive heating producing elements.
Self-regulating heating cables using PTC materials produce heat until the cable reaches a temperature limit essentially dictated by the switching temperature of the PTC material. The switching temperature is that temperature at which the resistance of the material rises sharply, often on the order of several orders of magnitude over a relatively short temperature range. The current flowing through the material decreases in response to the increased resistance, limiting the power output and preventing overheating.
As the cable temperature approaches the switching temperature, the resistive element's heat output will begin to diminish. The rate at which the heat output decreases is a characteristic of the PTC material used. For some materials, the heat output changes only gradually, while for others the change is more abrupt. The current will continue to diminish as the temperature rises, but will never completely terminate. A complete disconnection can only be achieved by cutting off the power supply.
PTC material may be used to form the heating element itself. For example, the heating element may comprise a PTC conductive polymer strip connected between the conductors. The heating element can also be a PTC ceramic chip. Alternatively, the PTC material may be connected in series with a heating element having a constant resistance, as shown in FIG. 3. In this case, the PTC material primarily controls the current to the resistor, and only secondarily acts as a heat producing element. In either case, the PTC material has a heat producing aspect which affects its performance. The current flow depends upon the temperature of the PTC material, which is influenced by the heating element's output as well as the temperature of its surroundings.
PTC materials can be subject to hysteresis effects. Some PTC materials behave differently when the cable is heating up than when the cable is cooling down. Consequently, the power on temperature of the cable can significantly differ from the shut off temperature. This disparity is generally undesirable and adds to design and manufacturing difficulties.